There is an increasing interest in producing energy, cellulosis, ethanol and other products from biomass. This includes, that the biomass undergoes pressurized processes, such as steam treatment, hydrolyzation, solvent extraction, pulping, explosion pulping, gasification, drying with superheated steam. The biomass can comprise of dry or wet particles or particles suspended in a liquid.
To achieve lowest possible production costs, it is crucial to establish reliable continuous processes and produce round the clock throughout the year.
Straw is a large biomass resource, which have not yet been intensively exploited, because its properties makes it very difficult to transport into, through and out of pressurized equipment. The main obstacles are:
Straw has a low density (loose shredded straw app. 50 kg/m3).
Straw is a non flowing product and has very strong bridging properties.
Straw has a high content of abrasive silicon.
These obstacles means that a method and apparatus able to handle straw in relation to pressurized equipment will be able to handle almost everything else such as woodchips, coal, residential garbage, by-products from slaughterhouses etc.
To be reliable the apparatus must meet the following requirements:                machine parts should only to a very limited extend “cut through” the product, in order to avoid wear and jamming.        the risk of bridging should be eliminated by forcing the product through the critical zones. This means that forced loading and unloading of sluice chambers are absolutely necessary.        it should be possible to compress low density products to a higher density in order to obtain a suitable capacity within reasonable dimensions.        
None of the known methods and apparatus based on sluice devices are meeting these requirements.
SE Patent 469 536 describes a chamber into which product is conveyed by a piston screw. At the inlet a cylinderknife slides forward and cuts through the product to close the inlet, but its function is to close for product and not to provide a pressure lock. At the outlet there is a pressure lock, but since there is only one, It is not a sluice device as previously defined. The apparatus is a plug flow feeder based on the ability of the higly compressed plug of product to reduce escape of gas when the pressure lock is open.
Rotary locks e.g. U.S. Pat. No. 5,114,053, where a rotor, comprising several pockets, rotates continuously in a cylindrical housing, demands a product with good flow properties. Machine parts have to “cut through” the product, which is problematic especially at the inlet. The product cannot be compressed and forced loading/unloading is not possible.
By DT Patent 24 26 035 a rotor with one sluice chamber turns intermittently, allowing the opening alternately to be connected to the high and the low pressure zones. A piston in the sluice chamber secures forced unloading of the sluice chamber and prevents emission from the high pressure zone. The product is not force loaded into the sluice chamber, therefore it can not be compressed, and machine parts have to “cut through” the product.
U.S. Pat. No. 5,095,825 describes a method where a rotor has two sluice chambers, which are force unloaded by pistons placed in the sluice chambers. The openings of the sluice chambers are placed in one end of the rotor, so that each of them will be connected to one of the two pressure zones when the rotor stops. The method seeks to reduce the risk of bridging during loading of the sluice chamber by creating a vacuum with the piston. This means, that the risk of bridging is only partly reduced if the product is penetrable for air. Machine parts would have to “cut through” the product and it is not possible to compress the product by this method.
By U.S. Pat. No. 5,819,992 a rotor with several parallel sluice chambers is used. The sluice chambers have inlet in one end and outlet in the other end. When the rotor stops for loading of one sluice chamber and unloading of another, tightness is established by expansion of dynamic sealing rings. When the loading/unloading operation is finalized, the dynamic sealing rings are contracted, thereafter the rotor can move to the next position with less friction but incomplete tightness. The method does not include portioning, so machine parts have to “cut through” the product. Furthermore the method does not include forced loading, option for compression or forced unloading.
SE Utläggningsskrift 456 645 describes a T-shaped sluice chamber, which forces the product to make a perpendicular movement from horizontal to vertical direction. The product is conveyed past the inlet pressure lock and into the sluice chamber by means of a piston or a piston screw, and thereafter the product has to fall by gravity only through the vertikal branch, until it lands on the outlet pressure lock. A separate piston secures forced unloading of the sluice chamber. The fact that the product during the loading of the sluice chamber has to make a 90° turn by means of gravity only increases the risk of bridging, and makes capacity increasing compression of the product in the sluice chamber impossible.
By U.S. Pat. No. 5,192,188 the product is loaded into the sluice chamber by means of gravity only, which gives a very poor filling. The discharge piston has to “cut through” the product at the inlet opening, and capacity increasing compression is not possible.
The advantage by the method according to the invention is, that it meets all the requirements for transfer of particulate, abrasive, low density, non flowing products between zones with different pressures.
To avoid “cutting through” the product the method according to the invention comprises a portioning device before the sluice device. The portioning device produces one or several sequences of uniform product portions divided by uniform particle free spaces. The particle free spaces secures, that no product particles occur in the working space of the pressure locks when they are closing.
To achieve force loading, the product portions are conveyed into the sluice device by means of a piston screw. The rotation and axial movement of the screw piston can be controlled independently which makes it possible to provide any degree of compression from light packing to transformation of the product portions into solid plugs.
Force loading and the possibility to achieve an adjustable compression of the product are very important features of the invention, because of the improved reliability and increased capacity that the apparatus according to the invention will achieve compared to the known apparatus. The known apparatus is designed to transfer particles of coal and wood with densities from 0.4-0.8 compared to 0.05 for shredded straw. This means, that the capacity on straw would drop to app. 10% if the volume of the sluice chamber was unchanged.
To achieve forced unloading of the sluice chamber according to the invention, different embodiments of the invention can be selected depending on whether emission from the high pressure zone during transfer of product from the sluice chamber to the high pressure zone is acceptable or not.
If emission is acceptable, as for example when the emission consists of steam, from which energy can be recovered by condensation, the piston screw which performs the forced loading can also perform the forced unloading of the sluice chamber. This implies that for each product portion transferred into the high pressure zone, a volume of steam will be transferred to the sluice chamber and further on to the place of condensation. For this situation pressure lock devices can be selected among well known valves such as slid valves, ball valves or piston valves. The inner diameter of the valves should at least be of the same size as that of the sluice chamber. Before opening the pressure locks, the pressure of the sluice chamber must be adjusted to establish substantially the same pressure at both sides of the pressure lock in order to reduce the power needed to open the pressure lock.
In special situations such as explosion pulping, where the product has to be discharged at high speed from the high pressure of the digester, the pressure of the sluice chamber should be maintained or even increased to accelerate the product to a very high velocity when the pressure lock is opened. For this special situation the diameter of the valve can be much smaller than the diameter of the sluice chamber, because of the high velocity of the product during discharge. A ball valve is a good choice because it can be opened completely in a very short time.
If emission is unacceptable for example when poisonous, explosive or malodorous gases are involved, the preferred embodiment of the invention comprises a rotor with two sluice chambers placed practically parallel to the axis of the piston screw, and either perpendicular or parallel to the axis of the rotor and equipped with pistons for forced unloading.
In this preferred embodiment of the invention the sealing system preventing gases vapours or liquid to leak out when product is transferred from the sluice chamber into the high pressure zone have to be resistant to the impact of the chemicals and the temperatures prevailing in the high pressure zone. By gasification for example the temperatures can be in the range of 700-1100° C. and process gases can contain considerable amounts of tare, which can condense at the much lower temperature of the sluice chamber. To avoid hot process gas to enter into the sluice chamber during unloading, it is known for example from U.S. Pat. No. 5,095,825 and DT 24 26 035 A1 to raise the pressure of the sluice chamber before unloading by means of pressurized inert gas. The supply of inert gas is however a substantial extra cost, and therefore a special sealing system has been developed to the invention, which practically eliminates leaking of process gas without utilization of pressurized inert gas at all.
The special sealing system comprises three sealing devices, which will be active at three different places.
The first sealing device comprises two sealing rings which will be active between the open ends of the sluice chambers and the outlet of the low pressure zone and the inlet of the high pressure zone during loading and unloading. This first sealing device is a known type of seal which can be extended to establish tightness during loading respectively unloading and contracted during movement of the rotor in order to avoid friction.
The second sealing device shall prevent escape of gas, vapour or liquid from the high pressure zone into the part of the sluice chamber behind the piston. Emission can occur when the sealing edges of the piston become worm, which is inevitable especially when silicia is present in the product. The second sealing device uses gas, vapour or liquid behind the piston, pressurized to substantially the same pressure as that of the low pressure zone during loading, and pressurized to the same or a higher pressure than that of the high pressure zone during unloading. The pressurized gas, vapour or liquid can also be used to move the piston during unloading.
The third sealing device comprises a vessel housing the rotor and is providing a sealed connection between the two pressure zones. This third sealing device will take care of any emission from the high pressure zone caused by wear or failures of the two other sealing devices. Any emission to the vessel will be detected and directed to a place where it will do no harm. The detection can release the action necessary to stop further emission.